[Dovecot] Error message Apache

I've setup Apache 2.2.9 for SOGo 1.3.2 on Debian 5.0.5. I know that the developers of Debian 5.0.5 are sometimes making choices which are not handy.

Which modules in Apache do I have to enable to make SOGo 1.3.2 working? That's what I understand from the error message here under.

Greetings, Frank

Proxy Error The proxy server received an invalid response from an upstream server. The proxy server could not handle the request POST /SOGo/so/test/preferences.

Reason: Error reading from remote server